The field of the present invention relates to planar waveguide substrates. In particular, heat sinks are described herein for facilitating heat dissipation on such substrates.
Planar optical waveguides are suitable for implementing a variety of optical devices for use in telecommunications and other fields. In addition to the planar waveguides, the planar waveguide substrate often also includes (by fabrication, formation, and/or mounting thereon): alignment/support structures for placement of optical devices on the substrate; V-grooves and/or other alignment/support structures for positioning of optical fibers and/or fiber-optic tapers on the substrate; compensators, gratings, and/or other optical devices on the substrate; electrical contacts and/or traces for enabling electronic access to active devices on the substrate; and/or other suitable components.
Silicon is a common substrate material for implementing planar optical waveguides, for a variety of reasons discussed further hereinbelow. For many examples of planar-waveguide-based optical devices, thermal conductivity of silicon substrates (typically single-crystal silicon substrates) is adequate. However, in certain instances the thermal conductivity of a silicon planar waveguide substrate may not be adequate for dissipating heat generated by devices and/or components on the substrate. In particular, FIG. 1 illustrates an example of an optical device 110 (on a device substrate 111 and including an external-transfer waveguide 112 in this example) surface-mounted on a planar waveguide substrate 102 for optical coupling to a planar waveguide 120 formed on the substrate. Active optical device 110 may be a laser or other optical source, an optical modulator, or other optical device or component that generates heat in the course of its operation. Substrate 102 may often include a low-index buffer layer 104 below waveguide 120. Alignment/support structures for positioning device 110 on substrate 102 are omitted for clarity. Substrate 102 is provided with an electrical trace and electrical contact 122 for establishing electrical continuity with corresponding contact 113 on device 110 after assembly (additional traces and/or contacts may be provided on substrate 102, or additional electrical access may be provided directly to device 110). In many active devices, relatively large amounts of heat may be generated in active regions of the optical device, particularly if relatively large drive currents are required. A primary route for dissipation of this heat is out into the substrate through the area of the electrical contact (indicated by the arrows in FIG. 1). For a variety of reasons, including providing high electronic bandwidth and conserving substrate area, the area of contact between the device and the substrate through the contact often may be made as small as practicable. The small area for heat dissipation and the moderate thermal conductivity of the silicon substrate may therefore result in inadequate heat dissipation and potential overheating of the device. It is therefore desirable to provide a planar waveguide substrate, particularly a silicon planar waveguide substrate, having enhanced thermal conductivity properties for dissipating heat from surface-mounted optical devices.